As a Knicks fan, I thought I'd give you my review on Q.
He's nothing special, but it's not fair to call him the worst player in basketball. He is very inconsistant, and really quite bad as a starter. He was, perhaps, the worst player in the NBA to start as many games as he did last year.
as a 35% shooter from 3 pt range, he will frustrate you. He's not the first guy you want sitting behind the 3 pt line chucking bricks. 35% isn't awful, but it's not good either.
Q used to have a pretty good post up game for a 6'6 guy, but he's had back problems over the last few seasons and that part of his game is gone. He's still a pretty good defender, and a smart guy on the court and good team player.
On days when he's on, you'll love him, but if he gives you what he gave the knicks, you'll dislike him more than you like him.
It's also possible that he was unhappy in NY, so he might play better in Memphis than NYC. I know that he didn't like playing with Marbury. I don't know how he felt last year, but it couldn't have been fun for him with all the fans preferring Chandler.
I think he's the kind of player a coach likes, because he does what's asked of him. I think that's why he played as much as he did, when the fans were screaming for his benching.
I'm not saying he's worth Darko, but maybe your coach sees something he likes in him. If Q can return a bit to his form in phoenix of LAC, he'd be a nice pickup for you guys, but I'm not sure that's likely. He does have back problems.
There's also a rumor that Q is gay, which would be kinda ironic, cause you'd have 2 gays on your team.

OK, ignore that.
Finally,
I wonder if this was the best deal you could have gotten for Darko, or if you could have gotten a 2nd rounder out of the Knicks in this swap. Either your GM likes Walsh, which happens sometimes that GMs are a little buddy buddy, or maybe your GM sees some value in Q.
You might not hate him as a player.